An oil well pump is an important constituent part of a pumping unit. Current oil well pumps usually have many segmented cylindrical metal barrels fitted in an outer tube and fixed by a cover at each end of the outer tube. A plunger is provided having a hollow barrel shape. A check valve is provided including a valve cover, a valve ball, and a valve seat. A pumping rod is connected to an upper valve cover and drives the plunger in a reciprocating motion, and when the plunger is in an up stroke, oil flows into the cylinder through a filter and the check valve. When the plunger is in a down stroke, the check valve at the bottom will be closed, and oil flows into an oil tube through a check valve to be lifted to the surface. A set of mandrels positions the cylinder to keep the segmented metal liner barrels co-axial. Then the cylinder is fixed in an outer tube by a sealing head at each end. There is some clearance between the liner barrels and the outer tube for convenience to adjust the positioning mandrels and the liner barrels. In order to prevent displacement of the liner barrels, when the positioning mandrels are removed, the upper sealing head and lower sealing head must be fastened down. In conventional practice, sometimes due to transport or disconnection of the pump, displacement of the liner barrels occurs, and the pump does not work normally. Although the liner barrels and the plunger have a hardened surface, there is friction between them causing them to wear. The presence of sand and hydrogen sulfide in the oil well will especially aggravate wear and corrosion. If the pump requires repair, the pump must be lifted from thousands of meters down the oil well, and a spare oil well pump will be set in. Not only is the replacement difficult, but it also is expensive. During the period of repairing, oil production will be decreased and the cost of oil production will be increased.